ISS Utilization: ISS-CREAM (Cosmic Ray Energetics and Mass)
Cosmic rays are energetic particles from outer space. They provide a direct sample of matter from outside the solar system. Measurements have shown that these particles can have energies as high as 105 TeV (or 1017 eV). This is an enormous energy, far beyond and above any energy that can be generated with man-made accelerators, even the Large Hadron Collider at CERN in Geneva, Switzerland.
NASA plans to place CREAM aboard the space station, becoming ISS-CREAM. The instrument has flown six times for a total of 161 days on long-duration balloons circling the South Pole, where Earth’s magnetic field lines are essentially vertical. 1) 2) 3)
ISS-CREAM is being developed as an international collaboration, including teams from the United States, Republic of Korea, Mexico and France, led by Professor Eun-Suk Seo of the University of Maryland.
Table 1: Some background on cosmic rays
Research overview: 4)
• Cosmic rays reach Earth from far outside the solar system with enormous energies well beyond what man-made accelerators can achieve. Space-based direct measurements of high-energy cosmic rays are difficult because of low particle fluxes in the most interesting regions, and there are no data sets with elemental charge resolution and adequate statistics.
• CREAM has accumulated ~161 days of data during six successful balloon flights over Antarctica. This longest known exposure for a single balloon-borne experiment verified the instrument design and reliability. The key value in residing on the ISS is the long exposure above the atmosphere and orders of magnitude greater statistics without the secondary particle background inherent in balloon experiments.
• One of the key discoveries CREAM made with balloon flights is spectral hardening, which contradicts the traditional view that a simple power law can represent cosmic rays without deviations below the "knee", around 3 x1015 eV, where the cosmic ray spectrum steepens. The current CREAM result provides important constraints on cosmic ray acceleration and propagation models, and it must be accounted for in explanations of the electron anomaly, which generated a lot of excitement in the science community, as well as the media, due to its possible dark matter explanation. CREAM on the ISS would greatly reduce the statistical uncertainties, and extend balloon-borne CREAM measurements, to energies beyond any reach possible with balloon flights. They will provide keys to understanding the origin, acceleration and propagation of cosmic rays.
Researchers are rearranging CREAM's existing hardware so it can attach to the Exposed Facility platform extending from Kibo, the space station's JEM /Kibo (Japanese Experiment Module), after its planned launch in 2014. The space station operates as a platform for instruments like CREAM that otherwise might not fly, due to the expense of dedicated satellites.
Figure 1: Schematic view of the CREAM instrument (image credit: NASA)
The ISS-CREAM instrument is configured with the CREAM calorimeter including carbon targets for energy measurements and four layers of a finely segmented SCD (Silicon Charge Detector) for charge measurements. These detectors have already demonstrated their capabilities to determine the charge and energy of high-energy cosmic rays from 1010 to >1014 eV for the proton to iron elemental range with excellent resolution. In addition, two new compact detectors are being developed: TCD/BCD (Top/Bottom Counting Detectors) and BSD (Boronated Scintillator Detector). The TCD and BCD each consists of a plastic scintillator and 400 photodiodes. As shown in Figure 2, the TCD is located between the instrument’s carbon target and the calorimeter, and the BCD is located below the calorimeter. These detectors provide the capability for electron separation from protons, a redundant energy trigger for the calorimeter, and a cosmic ray trigger for test and calibration on the ground. 5)
The hadron rejection power derived from the e/p shower shape difference can be significantly enhanced by making use of the thermal neutron activity at late (>400 ns) times relative to the start of the shower. Hadron-induced showers tend to be accompanied by significantly more neutron activity than electromagnetic showers. The ISS-CREAM BSD measures this late thermal neutron shower activity by detecting the boron capture of these thermal neutrons in a boron-loaded plastic scintillator (5% boron concentration by weight and the natural 10B abundance of 20%) located below the BCD under the calorimeter. Results from a 2012 beam test and the expected performance are discussed in another paper .
Figure 2: Exploded view of the ISS-CREAM instrument (image credit: ISS-CREAM collaboration)
Current results and expected performance:
One of the key results from the ongoing analysis of CREAM data is an observed spectral hardening for each element above ~200 GeV/nucleon, indicating a departure from a single power law. Proton and helium spectra in the energy range from 2.5 to 250 TeV are represented by power-law fits with spectral indices of -2.66± 0.02 and -2.58 ± 0.02 for protons and helium, respectively. Both spectra are harder than lower energy data from previous experiments, e.g., the AMS (Alpha Magnet Spectrometer) spectral indices of -2.78 ± 0.009 for protons and -2.74 ± 0.01 for helium. A broken power law fit for C, O, Ne, Mg, Si, and Fe with spectral indices γ1 and γ2, respectively, below and above 200 GeV/nucleon, resulted in γ1 = -2.77 ± 0.03 and γ2 = -2.56 ±0.04. As shown in Figure 3, the spectral index γ1 is consistent with the low energy helium measurements, e.g., the AMS index of -2.74± 0.01, whereas γ2 agrees remarkably well with the CREAM helium index of -2.58 ± 0.02 at higher energies.
A hardening of proton and Helium spectra around 240 GV, similar to the spectral hardening first reported by CREAM, has also been reported by PAMELA (Payload for AntiMatter Exploration and Light-nuclei Astrophysics), flown on Resurs-DK1 (launch June 15, 2006), using a permanent magnet spectrometer with a variety of detectors. The experimental uncertainties are too large to debate the exact starting point of the hardening, whether it is 240 GV or 200 GeV/nucleon. The exact cause of the spectral hardening is still under investigation, although a number of possible explanations of these results have been proposed. The hardening may result from modification of gas flow in the shock precursor by the cosmic ray pressure, which shapes the concave energy spectrum of cosmic rays. Alternatively, the observed hardening could be due to nearby sources, as suggested for the recent observations of an enhanced high-energy electron spectrum. A multi-source model by Zatsepin and Sokolskaya considered novae stars and explosions in super-bubbles as additional cosmic ray sources. Whether it results from a nearby isolated SNR (Signal- to-Noise Ratio) or the effect of distributed acceleration by multiple remnants embedded in a turbulent stellar association is another question.
Figure 3: Comparison of the high-energy spectra from a nominal ISS-CREAM mission (red circles) with existing data (black symbols), ISS-CREAM collaboration
Legend to Figure 3: Data from previous experiments include BESS (open squares), ATIC-2 (open diamonds), JACEE (X), and RUNJOB (open inverted triangles). Some of the overlapping BESS and AMS data points are not shown to achieve better clarity. The lines for helium represent a power-law fit to AMS (open stars) and CREAM (filled circles), respectively. The lines for C-Fe data represent a broken power-law fit to the CREAM heavy nuclei data: Carbon (open circles), Oxygen (filled squares), Neon (open crosses), Magnesium (open triangles), Silicon (filled diamonds), and Iron (asterisks).
Whatever the explanation, the CREAM results contradict the traditional view that a simple power law can represent cosmic rays without deviations below the “knee” around 3 x1015 eV. The pervasive discrepant hardening in all of the observed elemental spectra provides important constraints on cosmic ray acceleration and propagation models, and it must be accounted for in explanations of the electron anomaly and cosmic ray “knee”. Donato & Serpico reported that the spectral hardening reported by CREAM would lead to appreciable modifications for the secondary yields, such as antiprotons and diffuse gamma rays, in the sub-TeV range. They concluded that using a simple power law to model the astrophysical background for indirect dark matter searches, as often done in the literature, might lead to wrong conclusions about the evidence of a signal. Or, if a signal should be detected, use of a power law could lead to bias in the inferred values of the parameters describing the new phenomena. Yuan and Bi have demonstrated how tension between the AMS positron fraction and the total electron (including positron) spectra detected by Fermi and HESS can be removed by taking a harder primary electron spectrum at high energies, similar to the nuclei spectral hardening, for either pulsar or dark matter annihilation/decay scenario as the primary positron sources.
CREAM has pushed direct spectral measurements of nuclei, including the important secondary elements (e.g., boron), to ever-higher energies with Antarctic LDB (Long Duration Balloon) experiments. For primary element spectra, the energy region around 1015 eV is challenging to explore, because direct measurements run out of statistics at such high energies. Indirect ground-based measurements cannot resolve individual elements, and they encounter systematic problems caused by uncertainties in modeling hadronic interactions in the atmosphere.
ISS-CREAM can take the next major step to 1015 eV, and beyond. A 3-year exposure on the ISS would greatly reduce the statistical uncertainties and extend the CREAM measurements to energies beyond any reach possible with balloon flights, as illustrated in Figure 3. Being above the atmosphere, ISS-CREAM would be far superior to multiple balloon flights.
Status and plan:
The CREAM instrument is being reconfigured for accommodation on NASA’s share of the JEM-EF (Japanese Experiment Module - Exposed Facility) for at least an order of magnitude increase in the exposure factor. The scope of work required for the ISS investigation includes modification of instrument components for the ISS environment, in addition to assessing safety and mission assurance concerns. The instrument must be functionally tested and qualified to meet the launch vehicle and on-station requirements for operations on the ISS. The instrument needs to be repackaged within a structure that meets the JEM-EF interface requirements.
The basic design of the instrument is mature, and it has heritage operating over many years in the near-space environment. The radiation effects on electronic circuits need to be adequately addressed for ISS-CREAM. Components are selected and utilized in a manner to prevent the possibility of failures as a result of SEL (Single Event Latchup), and to assure that SEU (Single Event Upset) and SET (Single Event Transient) effects will have minimal impact on data collection. The issue of SEU could result in occasional corrupted data, and relatively infrequent reboots of the computer. The power supplies were designed with overcurrent trip circuits in the power distribution sections to rapidly remove power from any subsystem that exhibits a high current condition, which might be caused by a SEL. The instruments parts and components were evaluated for any destructive SEL failures by the Radiation Effects and Analysis Group at GSFC (Goddard Space Flight Center).
Replacement parts used to mitigate effects of space (e.g., radiation) were taken from NASA-approved parts lists and/or are undergoing rigorous environmental tests. Where the design includes FPGAs (Field-Programmable Gate Arrays), the control logics are being modified to use triple mode redundancy to mitigate errors caused by SEUs. Related software updates are being made, and development testing was conducted at the NASA/MSFC (Marshall Space Flight Center) in the Spring of 2013. The actual C&DH (Command and Data Handling) setup on the ISS was simulated by connecting the ISS-CREAM Science Flight Computer to the Payload Rack Checkout Unit. During the testing, reliable flow of commands and telemetry between MSFC and the Science Operation Center at the University of Maryland was established.
The ISS Program Office at NASA/JSC (Johnson Space Center) completed an ISS and launch vehicle accommodation study for ISS-CREAM. The ISS-CREAM payload is about the size of a refrigerator with a mass of ~1,300 kg, including government furnished equipment such as grapple fixtures and a PIU (Payload Interface Unit). The estimated ~600 W power and nominal data rate of 350 kbit/s are all within the available JEM-EF resources. ISS-CREAM utilizes an active TCS (Thermal Control System), a fluorinert fluid loop, provided by the JEM-EF through the standard PIU. Detailed thermal analyses of the ISS-CREAM payload are being performed.
ISS-CREAM is in its implementation phase to complete the detailed design, component fabrication, integration and testing of the fully integrated CREAM payload. As done for the ULDB (Ultra Long Duration Balloon) system flights, NASA/GSFC Wallops Flight Facility (WFF) is providing project management and engineering support for ISS-CREAM. Following environmental testing, the payload will be delivered to KSC (Kennedy Space Center).
Launch: A launch of ISS-CREAM instrument is scheduled for 2014 on a SpaceX service flight to the ISS. The launch site is Cape Canaveral, FL.
1) Dave Dooling, “ISS-CREAM to Tackle Century-Old Space Mystery,” NASA, May 30, 2013, URL: http://www.nasa.gov/mission_pages/station/research/news/iss_cream.html
3) E. S. Seo, T. Anderson, D. Angelaszek, S. J. Baek, J. Baylon, M. Buénerd, N. B. Conklin, M. Copley, S. Coutu, L. Derome, L. Eraud, M. Gupta, J. H. Han, H. G. Huh, Y. S. Hwang, H. J. Hyun, I. S. Jeong, D. H. Kah, K. H. Kang, H. J. Kim, K. C. Kim, M. H. Kim, K. Kwashnak, J. Lee, M. H. Lee, J. Link, L. Lutz, A. Malinin, A. Menchaca-Rocha, J. W. Mitchell, S. Nutter, O. Ofoha, H. Park, I. H. Park, J. M. Park, P. Patterson, J. Wu, Y. S. Yoon, “Cosmic Ray Energetics And Mass for the International Space Station (ISS-CREAM),” 33rd International Cosmic Ray Conference (ICRC), Rio de Janeiro, Brazil, July 2-9, 2013, URL: http://www.cbpf.br/~icrc2013/papers/icrc2013-0629.pdf
4) Eun-Suk Seo, “Cosmic Ray Energetics and Mass (CREAM),” NASA, May 6, 2013, URL: http://www.nasa.gov/mission_pages/station/research/experiments/1114.html
5) J. M. Park, T. Anderson, D. Angelaszek, J. B. Bae, S. J. Baek, J. Baylon, M. Copley, S. Coutu, M. Gupta, J. H. Han, H. G. Huh, Y. S. Hwang, H. J. Hyun, I. S. Jeong, D. H. Kah, K. H. Kang, H. J. Kim, K. C. Kim, K. Kwashnak, J. Lee, M. H Lee, J. T. Link, L. Lutz, A. Malinin, A. Menchaca-Rocha, J. W. Mitchell, S. Nutter, O. Ofoha, H. Park, I. H. Park, P. Patterson, E. S. Seo, J. Wu, Y. S. Yoon, “Results of Tests and Simulations for the Top Counting Detector and Bottom Counting Detector of the ISS-CREAM Experiment,” 33rd International Cosmic Ray Conference (ICRC), Rio de Janeiro, Brazil, July 2-9, 2013, URL: http://www.cbpf.br/~icrc2013/papers/icrc2013-1015.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.